To study the physiologic consequences of a genetic model of chronic hypervasopressinemia, transgenic rat was produced by induction of a mouse nietallothionein-human arginine vasopressin (AVP) gene into the germ line of rat. One stable transgenic pedigree was analyzed through several generations. Expression of the human AVP gene was demonstrated by Southern blotting and resulted in increased amounts of AVP in brains. Secretion of AYP from the central nervous system results in an increased concentration of the hormone in the plasma and in an increased excretion in the urine in amounts five to ten times compared to those of normal rats. Ectopic hormone production was found in the kidney. Transgenicrat showed no significant decrease in plasma Na level but decreased 24-hour urine volume under basal conditions. These data might suggest increased antidiuretic activity on kidney in transgenic rat.